The magnetoresistive effect is the change of electrical resistivity of a magnetoresistive (MR) material due to the presence of a magnetic field and anisotropic magnetoresistance (AMR) sensors comprise a MR material that sense the AMR effect. Magnetoresistance is expressed as a change in resistance ΔR due to the magnetization divided by the nominal resistance R. AR is a function of the angle between the magnetization and current direction flowing in the MR material.
The AMR sensors comprise a thin MR film. For example permalloy is about 80% Ni and 20% Fe, or a permalloy 80 film is roughly 80% nickel, 15% iron, and 5% molybdenum. As used herein, %'s of materials are all weight percentages. There is a hard axis with a high requirement of magnetization energy in one direction in the plane of the magnetic film, and orthogonal to the hard axis in the plane of the film is an easy axis which indicates the magnetic preference direction. AMR sensors offer robust non-contact measurement of changes in the magnetic field as seen by the sensor, where the AMR sensor can detect the presence, the strength, and/or the direction of the magnetic field.
In typical AMR sensor applications, MR materials are ferromagnetic materials such as nickel iron (NiFe) which have top side electrical connections that are used to detect the component of a magnetic field that lies in the plane of the MR material. In some applications, the MR material is disposed in a serpentine array on the surface of a substrate such as silicon. The serpentine pattern of MR material can be electrically connected in a Wheatstone bridge arrangement (4 resistors) or a pair of Wheatstone bridges in order to sense changes in the resistance of the MR material resistors in response to changes in the strength of a magnetic field component in the plane of the MR elements. In order to monitor the changes in the resistance of the MR material resistors, associated components such as amplifiers that are on the same chip are generally connected together to form an electrical circuit which provides an output signal that is representative of the strength of the magnetic field in the plane of the MR sensing element resistors.
Designers of AMR sensors purposely select MR materials that have a magnetostriction that is as low as possible, because the magnetostriction effect can change the measured resistance leading to sensing errors. The exact composition of the MR material alloy may be selected to try to obtain a 0 value for magnetostriction, such as about 81.5% Ni. For example, a conventional permalloy film (about 20% Fe and 80% Ni) although having a relatively high magnetoresistance value, generally has a magnetostriction coefficient (λ) that is <<1 per million (ppm).